1. Field of the Invention
The present invention generally relates to optical receivers, and in particular to automatic offset control for an optical receiver for receiving a burst mode optical signal.
2. Description of the Related Art
There has been an optical receiver without the use of coupling capacitor, which is generally called a dc-coupled receiver. Although such a dc-coupled receiver is ideally suitable for burst mode operation, it is very important to solve the offset problem stemming from faulty or deteriorated extinction ratio and reflected return light at sending side and a dark current flowing through a photodiode at receiving side. If such an offset current is not compensated for, the receiving sensitivity of the receiver would be impaired.
To compensate for the offset, an automatic offset canceler (AOC) has been proposed (see Japanese Patent Unexamined Publication NO. 5-218772). In the AOC, a differential amplifier produces a positive output signal and a negative output signal from an input signal. Positive and negative peak detectors receive the respective positive and negative output signals to produce positive and negative peak voltages which are used to produce an offset adjustment signal based on a difference voltage thereof.
Another conventional AOC has been disclosed in U.S. Pat. No. 5,539,779 (issued on Jul. 23, 1996). This conventional AOC is comprised of a differential preamplifier, an average detector, a peak detector, and a differential amplifier. The average detector detects an average voltage of a positive output signal and a negative output signal of the differential preamplifier. The peak detector detects a peak voltage of the negative output signal of the preamplifier. The differential input amplifier compares the peak voltage with the average voltage as a reference value to produce an offset adjustment signal. In other words, the offset adjustment signal is obtained based on a difference between the average voltage and the peak voltage.